X-ray tubes having rotatable anodes or anode rotors are commonly and widely used in the X-ray field. The rotating anode provides a constantly changing target area for the generated X-rays so that the heat generated by the X-rays is more effectively dissipated. Such heat dissipation permits higher energy levels to be utilized thereby resulting in an increased X-ray current output as compared with tubes having a fixed anode.
IN SUCH ROTATING ANODE TYPE OF TUBES, IT IS NECESSARY TO PROVIDE A MEANS TO ASSURE THAT THE ANODE HAS ATTAINED A SELECTED ROTATIONAL SPEED PRIOR TO THE APPLICATION OF HIGH VOLTAGE POWER TO THE ANODE AND THE SUBSEQUENT GENERATION OF X-rays. Various prior art systems utilize time dependent apparatus for accelerating the anodes, and after a selected time initiating the X-ray exposure. In such prior art devices, a boost or acceleration signal is applied to the anode to start the anode rotating, and the generation of the X-rays and the bombardment of the anode is delayed for a fixed period of time to enable to anode to reach a selected rotational speed before the X-rays bombard on the anode. However, in boosting the speed of the anode to the desired speed, the prior art relies on boost power which is applied to the associated stator for a pre-selected fixed time and there is no other control to assure that the tube is actually rotating at the selected desired speed at the termination of the time period of the boost power.
Problems arise, since as a tube is used repeatedly, the associated mechanical and support assembly heats up and more resistance power is dissipated in the stator windings. As the tube housing gets hotter, the associated stator loses efficiency since its resistance goes higher, while the inductance remain fixed and causes more power to be dissipated in the form of heat in the windings and less power is transmitted to the anode rotor. Thus, it has been found that after the tube gets hot, most stators do not receive the required amount of energy to drive the anode to attain the desired running speed within the normal fixed boost time period.
Another problem of rotating anode tubes is the vibration caused by the mechanical resonance of the anode system. In presently available tubes, the mechanical resonance of the system is at approximately 6,000 RPM. Accordingly, it is desirable that the anode be accelerated and decelerated through this mechanical resonance point in a minimum of time to reduce any wear and damage caused by vibration as the anode speed goes through this point.
Another type of prior art comprises a mechanical vibrating reed tachometer. Frequently, the amplitude of mechanical vibrations of the rotating anode is not enough to excite the vibrating reeds at all running speeds. Tubes are normally designed to give minimum vibration at their desired operating speed, and it is extremely difficult to measure the vibration particularly at the low operating speeds.
Another type of prior art consists of a tachometer approach including a light which shines upon the rotating anode and a marker applied to the anode which reflects light at each revolution of the anode. The information is reflected to a light sensing device to develop an electrical signal, proportional to rotational speed of the anode. A disadvantage of this type of device is that markers must be placed on each anode; and, further, photo-optic circuits are often unreliable.
An important advantage of the inventive system is the provision of signals indicating selected rotating speeds of the anode for enabling control signals to be generated responsive thereto, to thereby control the acceleration and the deceleration of the anode rotor. More specifically, the inventive system senses vibrations of the mechanical housing of the rotating tube, translates that to a frequency related to the rotational speed and converts that into an electrical signal which is processed to provide a control signal.
The present invention conveniently obtains a signal from a position external to the tube. The present system thus does not require cutting into the electrical leads which supply power to the associated split phase motor which is used to drive the anode, and does not otherwise require any modification of presently available X-ray tubes.
An advantage of the inventive circuit is that it can be connected in the circuit or inter-locked to insure that the boost or accelerated signal continues to be applied until the tube is rotating at the desired speed before X-rays are directed to bombard the anode, thereby protecting the anode from destruction or damage. Also, the present circuit can assure not only that the tube is brought up to speed during the boost period, but also that the tube remains at that desired speed during the entire X-ray exposure period. For example, if the speed of the tube is reduced for any reason, such as by a mechanical lock-up or electrical power reduction, the present circuit would deactivate the X-ray sources.
During the boost periods when the X-ray starter is powered to accelerate the anode, the filament is brought up to high brightness, high current level in anticipation of the X-ray exposures. The longer the time required to boost the tube to the proper operating speed, the longer the filament must remain on high brightness condition. Thus, another advantage is that the circuit provides a signal such that the filament can be energized to a high brightness condition only for the time period required; the foregoing tends to extend filament life.
Another advantage of the present invention is that by putting the inventive circuit as a feedback control, the amount of boost time required on every exposure is reduced. For instance, if the tube is coasting at 2,500 RPM and a boost signal is provided, a minimal boost period is required to accelerate the tube to the selected speed, say 3,000 RPM, to prepare it for X-ray exposure.
Another important advantage of the present circuit is that a tube should decelerate rapidly from 9,000 RPM to a point below 6,000 RPM to minimize the effects of the tremendous vibration which results at the mechanical resonance of the tube which is at about 6,000 RPM. Normally, the anode is accelerated to a high speed, and after X-ray exposure, the anode is braked for a pre-selected fixed time to decelerate the anode quickly through 6,000 RPM point so as to minimize vibration damage to the filament and associated structure.
The present circuit will assure that braking is done properly and that the anode does not coast down through the damaging mechanical resonance point.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings wherein: